Traditionally, telephony communications within the United States were handled by the public switched telecommunications network (PSTN). The PSTN can be characterized as a network designed for voice communications, primarily on a circuit-switched basis, with full interconnection among individual networks. The PSTN network is largely analog at the local loop level, digital at the backbone level, and generally provisioned on a wireline, rather than a wireless, basis. The PSTN includes switches that route communications between end users. Circuit switches are the devices that establish connectivity between circuits through an internal switching matrix. Circuit switches set connections between circuits through the establishment of a talk path or transmission path. The connection and the associated bandwidth are provided temporarily, continuously, and exclusively for the duration of the session, or call. While developed to support voice communications, circuit switches can support any form of information transfer (e.g., data and video communications).
In a traditional PSTN environment, circuit switches include central office (CO) exchanges, tandem exchanges, access tandem exchanges, and international gateway facilities. Central offices, also known as exchanges, provide local access services to end users via local loop connections within a relatively small area of geography known as an exchange area. In other words, the CO provides the ability for a subscriber within that neighborhood to connect to another subscriber within that neighborhood. Central offices, also known as end offices, reside at the terminal ends of the network. In other words, COs are the first point of entry into the PSTN and the last point of exit. They are also known as class 5 offices, the lowest class in the switching hierarchy. A class 5 telephone switch communicates with an analog telephone using the analog telephony signals in the well-known analog format. The class 5 telephone switch provides power to the telephone; detects off-hook status of the telephone and provides a dial tone in response; detects dual-tone multi-frequency signals from the caller and initiates a call in the network; plays a ringback tone to the caller when the far-end telephone is ringing; plays a busy tone to the caller when the far-end telephone is busy; provides ring current to the telephone on incoming calls; and provides traditional telephone services such as call waiting, call forwarding, caller ID, etc.
In an effort to increase the amount and speed of information transmitted across networks, the telecommunications industry is shifting toward broadband packet networks which are designed to carry a variety of services such as voice, data, and video. For example, asynchronous transfer mode (ATM) networks have been developed to provide broadband transport and switching capability between local area networks (LANs) and wide area networks (WANs). The Sprint ION network is a broadband network that is capable of delivering a variety of services such as voice, data, and video to an end user at a residential or business location. The Sprint ION network has a wide area IP/ATM or ATM backbone that is connected to a plurality of local loops via multiplexors. Each local loop carries ATM over ADSL (asymmetric digital subscriber line) traffic to a plurality of integrated service hubs (ISHs), which may be at either residential or business locations.
An ISH is a hardware component that links business or residential user devices such as telephones and computers to the broadband, wide area network through a plurality of user interfaces and a least one network interface. A suitable ISH is described in U.S. Pat. No. 6,272,553 entitled “Multi-Services Communications Device,” issued on Aug. 7, 2001, which is incorporated by reference herein in its entirety. The network interface typically is a broadband network interface such as ADSL, TI, or HDSL-2. Examples of user interfaces include telephone interfaces such as plain old telephone system (POTS) ports for connecting telephones, fax machines, modems, and the like to the ISH; computer interfaces such as B ethernet ports for connecting computers and local area networks to the ISH; and video ports such as RCA jacks for connecting video players, recorders, monitors, and the like to the ISH.
In providing telephony services over a broadband network, the ISH communicates with a service manager. This connection between the telephone and the network element is typically an ATM connection, which is much different than the traditional analog line to the local switch. ATM connections usually do not support analog telephony signals, such as off-hook, dial tone, and busy signals. Therefore, the ISH must provide many of the telephony functions traditionally provided by the telephone provider central office such as detect off-hook conditions, on-hook connections, and digits as well as provide the telephones with dial tone, ring voltage (sometimes referred to as ring current), ringback, and busy signals. The terms off-hook and off-hook condition as used herein are generic terms meaning that a user device (whether telephone, facsimile machine, modem, etc. connected to a telephone line is attempting to access and use the line.
The ISH includes a processing core or central processing unit, CPU, which controls these functions. It must exchange data with numerous peripheral devices within and external to the ISH itself. It is desirable to use the Universal Test and Operations PHY Interface for ATM level 2 (“UTOPIA 2”) protocol for some of these data exchanges. This protocol was developed by the ATM Forum and is an accepted industry standard. In one aspect of this protocol, data is transferred from multiple peripheral devices (called PHYs, an acronym for physical layer interfaces) to the ATM layer device, such as the central processor, on a read bus and from the ATM layer to the PHYs on a separate write bus. Address and control buses are also provided for controlling the data transfers between the ATM layer and the various PHY devices. The ATM layer interface controls data transfer by generating commands or signals indicating when it is ready to read or write data.
There are two particular functional aspects of this polled multi-PHY mode of UTOPIA 2 that are relevant here. In the first aspect, the ATM layer ‘polls’ the available PHY devices using the address bus and the Cell Available indication (CLAV) status line to discover which PHY devices are ready to transfer a cell. The second aspect is when, using accumulated knowledge of which PHYs need service, the ATM layer will, in a later cycle, use the same address lines in conjunction with other control lines to select a single PHY to initiate a cell transfer. For example, when the ATM layer needs to read data from a peripheral, it drives the read address bus with the address of the selected peripheral and places a read enable command on the control bus. In response to these commands, the addressed device writes, or drives, a byte on the read data bus during each of a series of clock cycles. On the positive going transitions of each clock cycle the CPU reads and stores the byte which was supplied by the peripheral during that clock cycle.
The UTOPIA 2 protocol defines all this ATM layer/PHY interaction, but it was prepared with the assumption that the controller and all peripheral devices would physically reside on the same printed circuit board. The read and write buses would therefore be short and the controller and peripherals would have sufficient drive capacity on their data outputs to directly drive the buses. Generally the controller and the peripherals are in the form of integrated circuit chips mounted on a printed circuit board. The protocol did not anticipate the complex systems like modern telecommunications hubs which have the ATM controller mounted on one circuit board and peripheral devices carried on separate printed circuit boards. The multiple boards are interconnected by being plugged into a backplane to receive operating power, address and control signals, and for connection to the read and write buses. Peripheral devices do not generally have sufficient power to drive the signals onto and across the backplane buses which are physically much longer than those anticipated by the chip designers. In addition, the backplane operating environment may not be appropriate for the integrated circuit interfaces, such as for hot-swap.
While it is known to use buffers to increase the drive capacity of a signal, the UTOPIA protocol does not anticipate such use. The peripheral chip designers also did not anticipate such use. The peripheral devices receive the control signals from the controller and decode these internally to know when to send or receive status and/or data signals. When the outputs of a peripheral are not sending data, they go to a three state or high impedance output condition. This allows other peripherals connected to the signal wires to drive signals on the bus. However, the peripheral devices do not provide any output signal indicating when it is driving data and when it is in three state condition. If buffer inputs were connected to such data outputs, the buffer would go into an unknown state and possibly oscillate when the peripheral outputs go high impedance. Its outputs would continue driving the backplane, which is not acceptable. It is desirable to provide a system which allows the outputs of a peripheral device to be buffered to drive data across a backplane bus without these problems.